Thank you for the replies I appreciate it.
Now I have 3 choices:
Sapphire HD5570,5770,5870.
Gonna check which among these video cards can run on a generic 600 watts PSU.
I've done a quick research about the 5570 and it could run on my psu that's why it's my first choice.
By the way yes I will also upgrade my ram and I am already running on Windows 7 64 bit.